Did they receive goods or services in exchange for the payment?  

Someone wants to deduct part of their daughter's wedding costs?  No.  And not the stipend to the clergyman, either.  

It was for a wedding which was cancelled. My client "gave" the rented venue to their church for use for the church's fund raiser. The church provided my client with a donation receipt for flowers, food, etc. that was given with the use of the venue. The venue (owned by a 501c(3) foundation) provided my client with an invoice showing the amount billed for rental, nothing about it being a donation.

"with an invoice showing the amount billed for rental, nothing about it being a donation."

Because it was Not a Donation. It was Rent Payment. Then, your client gave that right to the Church, so there is no Donation to the venue; there is Payment to venue as rent.

What you have here is Church Fundraiser activity. The Church didn't give them anything showing that rental amount, but your client has proof of the value they gave, as a PrePaid Venue.

The venue is not a party to any of that part between your client and their Church.