Unless there is a specific provision in your standing orders, no councillor can make unilateral decisions, not even the the Chair (except to call an Extraordinary or Extraordinary General meeting) .
Normally, the Clerk should have satisfied himself (or herself) that the spending had been subject to a resolution.. However, we don't know if the Cllr decided to bypass Standing Orders and regs deliberately or out of ignorance. For that reason it would be justifiable to ask the Cllr to clarify this at a full meeting of the council.