Paying the value of gold sometime after concluding the sale contract
If the case is as you have mentioned;
that paying the value of the gold is after a period of concluding the sale contract, then this is not permissible if the deferred value is gold or silver, or any other thing of the same value such as banknotes. This is a form of Riba Al-Nasi'ah (usury of delay, conditional excess for delay of payment). However, if the value is of another kind such as wheat, cloth, iron or the like, it is permissible.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.