BUSY has a utility which auto create and adjust bill references on FIFO basis. To know how to use it, follow these steps:
Step 1: Go to Administration then click on Utilities.
Step 2: Select Reference Management and click on Auto Adjust References.
Step 3: Select Bill wise References and can select either All Accounts, Group of Accounts or One Account.
Step 4: Press Ok to run the process. This will auto create and adjust reference on FIFO basis.
Note: Take backup of your data before running this process.
Further, if you need any training you can contact to your authorized channel partner.