Just to add my two cents:
Say I accepted a shop that where part of the shop is payment for the shop and reimbursement to a certain amount for expenses incurred.
Where the money is derived for the expenses incurred is not the business of the MSC (unless, I suppose it was stolen).
Examples:
1) I did something for someone and they said they would pay for it via their cash/credit card.
2) I was nice to someone and they in turn bought something for me with their cash/credit card.
3) I found some cash while on another shop. The original owner could not be found. I used it to pay for the expenses incurred. Not so far-fetched - I found as much as $60!
4) I was short of funds, my significant other, friend, mother, father, sibling gave me their credit card to pay for the expenses incurred.
There are more scenarios. The point is, none of them are the MSC's business as to how the expenses incurred were paid. The shop was performed, expenses were incurred and should be paid by the MSC up to the amount's maximum as outlines in the shop's guidelines.
That's my take.
PS, I am guilty of using my sig other's CC (with permission) for shop expenses. In turn sig other received a "service" from me as compensation.