Nonmerchandise partner invoice with PO Number

RMS/ReIM 14.0.2

I'm trying to do following scenario:

I created Cost component - type Expense. Mapped it no Non-merchandise Code in RMS.

Item with this cost component is added to the PO. PO is received.

My total merchandise cost is 1000, and my expense is 100; the total ELC is 1100.

In ReIm I create merchandise invoice for 1000 and match it - this works fine.

Since I'm using Simplified RTM, there is no obligations in RMS, so in ReIM I'm trying to create non-merchandise invoice for partner and assign order number.

ReIM does not allow entering the Order number for non-merchandise unless Vendor type is supplier. After entering some other fields (document date, due date), Order no field is available, but it does not allow to enter the PO - even when partner is added to PO in Order Attributes screen.