Does borrowing money from a fiancée to pay her dowry before conducting the
marriage contract invalidate the marriage? Please note that the husband paid the debt some time after the marriage. Is recording the value of the dowry in the marriage contract, and which the groom borrowed from his fiancée without her guardian's knowledge, considered deceit and thereby invalidate the marriage contract? And if so, how can the contract be rectified? The bride's guardian claims that the marriage contract has been invalidated and stipulates a divorce to rectify it.

The dowry, whether paid at the time of marriage or deferred, is valid. Borrowing the value of the dowry does not invalidate the marriage. It makes no difference whether the debtor is the one to marry the bride or any other and this is of no consequence on the validity of their marriage at any time in the future. Recording the dowry (which is a debt upon the groom) in the marriage contract does not involve deceit in any manner. It is not right to claim that the contract is invalid — if this were the case then all the marriages involving deferred dowries would be considered invalid.

The ruling
The condition set by the bride's guardian to rectify the contract is inconsequential because the contract is valid in the first place.
Allah the Almighty knows best.