Several options:
The simplest would be to allocate the supplies to Jobs at the time of purchase by splitting the vendor vendor bill into multiple lines so you can code the appropriate amount to each Job. Likely you dont know in advance or youd already do this.
Next would be to leave the Job codes off the vendor purchase and create additional entries (cheque, bill or JE) later to realloacte into Jobs when you do know. This would be a net zero entry that just posts a supplies expense with the job code, and then reverses that expense without any job code.
Third option is to start using inventory items for these supplies, and buy the stock initially unallocated, then later 'sell' it into the jobs as it is used.
