Please note: I am not a lawyer.
I will assume you are in the United States.
In the event of a discrepancy between the written and numerical amounts on a check or other negotiable instrument , the written amount legally takes precedence over the numerical amount .
The Uniform Commercial Code states: § 3-114.
Contradictory terms of instrument.
If an instrument contains contradictory terms, typewritten terms prevail over printed terms, handwritten terms prevail over both, and words prevail over numbers.
As such, the check should be processed as $50.00, not $25.00.
It is technically valid, but the discrepancy could cause it to be rejected by the bank when it is deposited.
money.stackexchange.com/users/19932/bwDraco
I gather you are from the US, as you spell it check whilst in Australia
we spell it
cheque, but I am sure the rules should be the same in Australia as in the US in regargds to your question.
If the word and number amounts are different the bank or financial institution won't accept the check, so you either have to get the person who issued and signed the check to correct one of the amounts so that both amounts are the same and initial the correction, or else get them to send you a new check with both amounts matching and destroy the first check.
You will probably have less troubles though with a brand new check.
money.stackexchange.com/users/5466/Victor
This answer is based on my recent experience (past 6 months) depositing a check in the U.S.The check in question was mismatched such that the spelled-out amount was approximately $50 less than the numeric amount.
The check was deposited via an online transaction with a scanned image.
The discrepancy was not caught by the bank until a few days later and I received an email notifying me of the error.
The check was accepted by the bank, however at the amount spelled-out in words.
I spoke personally with a bank representative who informed me that the check was indeed valid as written but the spelled-out amount takes precedent.
This seems like a bizarre protocol to me as it will almost surely result in some party experiencing an unexpected transaction amount.
That being said, and to answer the question, if the spelled-out amount is correct you should let the other party know about the discrepancy, let them know you intend to deposit it that way, and go ahead an deposit it making sure to check (no pun intended) with your bank that they'll accept it as-is and not accuse you of fraud.
money.stackexchange.com/users/5963/MSlimmer
A new cheque is suggested.
In many countries, in case of mismatch amount in word is accepted.
In few countries, these cheques are rejected.
Suggest, get a new cheque.
If you have to get the old cheque initialed, you will have to send them the cheque and get it back.
BTW, in India, we too write cheque and not check.
money.stackexchange.com/users/5334/Natwar Lath
A check or draft must describe a certain sum of money to be paid to the drawer (the person presenting it).
If there is ambiguity about the amount of money to be paid, it is not valid.
money.stackexchange.com/users/1288/duffbeer703
In the United States, according to the Uniform Commercial Code, when there is a difference between the words and the numerals, the words are what counts.
money.stackexchange.com/users/35712/Mark
I received a check personally written to me from a family member.  The family member that wrote & sent me the check wrote where you write out the amount of what the check is suppose to be for, in words, "Fifty Dollars-------00/00".  Then, in the box above where you write the amount with numbers, they wrote "25.00".  They signed the check and mailed it to me to cash.  What should I do?
What if I received a check with two different amounts written on it (words vs. number)?
money.stackexchange.com
<check><mistakes>
money.stackexchange.com/questions/13372
money.stackexchange.com/users/5678/AMK
The written amount legally takes precedence, however the bank might not accept the check if the amounts are different. Where there are contradictory terms, typed terms prevail over printed, handwritten over both, and words prevail over numbers. Where there is a discrepancy between written and numerical amounts on a check, the written amount legally takes precedence over the numerical amount. If the word and number amounts differ, the bank or financial institution may not accept the check and you will have to ask the person who issued and signed it to correct one of the amounts so that both are the same and initial the correction, or ask them to send you a new check with both amounts matching.
Where there are contradictory terms, typed terms prevail over printed, handwritten over both, and words prevail over numbers. Where there is a discrepancy between written and numerical amounts on a check,  the written amount legally takes precedence over the numerical amount. If the word and number amounts differ, the bank or financial institution may not accept the check and you will have to ask the person who issued and signed the it to correct one of the amounts so that both are the same and initial the correction, or ask them to send you a new check with both amounts matching.