How to use "prospective offer" in sentence

Given the standard meaning of prospective - "relating to or effective in the future" ( http://www.merriam-webster.com/dictionary/prospective ) and the way it's usually used, I don't think you can use the term prospective offer gracefully in any way but as a reference to something in the past. "We made a prospective offer" sounds fine. "We are making a prospective offer" sounds clunky and somehow wrong.

Are you sure that you aren't making a contingent offer? That is one that becomes effective if certain conditions are met rather than one that simply exists in the future.

Following are three examples which use prospective offer to mean an offer mentioned or discussed but not yet tendered.

...you should have your lawyer review your prospective offer and contract documents before you make an offer. [Real Estate Investing For Canadians For Dummies® by Douglas Gray, Peter Mitham, at books.google]

I gently began discussing a prospective offer with an elderly owner lying in her bed quite dead. (The buyers were right behind me as I spoke). [Alby Goldman]

The real estate agent called [about] bringing an offer to the table. [...] He decided to tell her about the prospective offer. [On the Street Where You Live by Mary Higgins Clark, in books.google]

I would see a prospective offer as one that has not yet been made formally. It may be conditional (scholarship offer that is subject to performance in your current school) or something you have heard at second hand.

I think you intended that the offer is not being tendered formally or in a binding way. If that is the case, you might want to say:

We would like to make a tentative offer for...