The original order is probably that the mother first consecrated the silver to Yahweh, and only then did the son own up to the theft — the consecration of the silver likely being a ploy by the mother to get the son to own up to the theft (particularly as she only puts a fraction to the purpose it was consecrated for).
The original plan was for the mother to first consecrate the silver to Yahweh, and then have her son admit to stealing it. The mother's action of only using a portion of the consecrated silver suggests that she may have used this as a way to get her son to confess.