I think Mr Bennet would have made his utmost to repay his brother in law if he had really paid for the marriage.

He ponders the situation over during most of ch. 49 (contrary to foolish Mrs Bennet ( "it is all very right; who should do it but her own uncle? If he had not had a family of his own, I and my children must have had all his money, you know;).
Later in ch 59 Mr Bennet says : And so, Darcy did everything -- made up the match, gave the money, paid the fellow's debts, and got him his commission! So much the better. It will save me a world of trouble and economy. Had it been your uncle's doing, I must and would have paid him;