Could you just clarify…..
When you say “we” run an allotment association….
Do you mean you are a member of an allotment association? Are YOU a Cllr on the PC and an allotmenteer or just a party to the allotment association?
It is entirely inappropriate for an allotment association to have a direct liaison with a single Cllr - whether trusted or otherwise. No individual Cllr should act other than directly delegated by the PC and even then not as a single point of authority - that actually IS the role of the clerk
The allotment association should have a ‘relationship’ with the whole corporate body of the PC via the proper officer.
It is the clerk that should be issuing invoices / checking payments (if that is the arrangement) but certainly not an individual Cllr.