UOM mismatch between PO and Invoice

Hello Gurus,

We have not implemented inventory and create only expense POs. 

PO is being created with UOM of "Feet" and received using "Feet". But the supplier is sending the invoice with UOM in "Spool/Roll".

Eventhough the extension amount of PO and Invoice, AP team denies to pay the supplier due the mismatch of UOM between PO and Invoice.

We have multiple suppliers supplying the same part. It is getting hard to convince the suppliers to use the same UOM.

Is there any workaround / solution to this issue?

Thanks in advance,

MS