Question

Why does Mr.Loisel give his wife all of his savings for a dress?

Answer 1

Explanation:

he sacrifices his hunting rifle he's spent months saving for so Mathilde can buy a dress for the ball. he's very devoted to his wife. he does anything for her

Answer 2

I would argue a guilt-ego complex. At the time husbands were given the roles of being the ultimate providers for their wives, material items or other. Thus their financial situation would’ve reflected poorly on him specifically and made him feel as if he wasn’t fulfilling his role as husband. This was his way to compensate or ‘make it up’ to his wife because he felt that he was failing her as a husband and was desperate to please her for once.

You also just say he loves her, but that could also link to the above.