You can use the stock receipt (Stock > Inventory Management > Receipts > Add Goods Receipt) with negative values to reduce stock without affecting sales records. It does require a matching stock order to keep backorders neutral. You would need at least one dummy supplier in the system and you will get good reporting from the E4 Purchase Report.
The process is quite quick. You would use Stock > Inventory Management > Receipts > Add Goods Receipt, select the required dummy supplier and use a blank sheet to generate an order using negative amounts.
When complete, process the order and immediately receipt it using Stock > Inventory Management > Receipts > Add Goods Receipt. Use the same dummy supplier, make up an invoice number and when the receipt opens, process it unchanged.
This method gives you good reporting and does not require any balancing transactions on financial records.
For record keeping, you could have just one dummy supplier called (e.g. Internal Use) and use names such as Vehicle 1, Vehicle 2, Branch Clinic etc. instead of invoice numbers when you create the receipt.
However, if you were to create dummy suppliers called Vehicle 1, Vehicle 2, Branch Clinic etc. the E4 report will sub-total the values for each of these centers. You will have to think of some other values to use for supplier invoice numbers when you process the receipt.
Again you can use further negative quantity order/receipts for seasonal variations and positive value order / receipts to transfer stock back to the main hospital.