In theory, this should be simple enough. A councillor has no power to make decisions for the council. The clerk may have delegated powers, set by the council, but otherwise can only implement council decisions. Council decisions can only be made in a properly constituted meeting.
A councillor can be appointed clerk, but this must be done by a council decision. A councillor cannot receive payment from the council while a councillor or for a year afterwards. A councillor acting as clerk should be very careful to keep separate the role of clerk and that of councillor.
In practice, things rather depend on the exact circumstances. If a council account has been used, it may be difficult to repudiate the transaction. If the council wants to keep whatever was purchased, it would be unreasonable not to pay for it. If, however, the councillor has made a purchase at their own expense, then the council has the option of refusing to accept the transaction and leaving the councillor to extricate themselves.