This goes back to the poor sailor caught between Scylla & Charybdis.
To support another proposal, for which I need only write a letter, I have written:
I am tremendously excited about the potential for us to develop our collaboration on the neurophysiology of bunny hopping. We have discussed this project at length, and I am impressed with the design in your proposal.
To facilitate our collaboration, I invite you to visit my laboratory in the next year. Your proposal to collect the pilot data necessary for our collaborative project is strong.
Let me dissect a few things I think important:
the potential for us to develop our collaboration
The point here is to make it clear this is a joint project. I see this as an "us, rather than "me help you with your project". The funding agency needs to hear that I am committed to this.
We have discussed this project at length, ....
Again, a bunch of "we"s in the opening para.
But, moving on
impressed with the design in your proposal.
and, in the second paragraph:
Your proposal here to collect the pilot data necessary for our collaborative project is strong.
It is now time and important to show that the collaborator is doing the heavy lifting. They are the PI, not me. I did not write this document, the colleague did. Make that clear.
Later on in the letter I wrote:
I am excited at the potential of expanding my studies in bunny hopping to include your expertise in defining more anatomically correct feet.
Actually, I didn't write this, my colleague did in a draft. But its important to say "I am getting something out of this", which emphasizes my commitment to the project.
The rest of the letter is the usual "I have all the facilities necessary to study bunny hopping in situ". All paired appendages crossed....
ps - the grant we wrote was superb and is at my grants office, and on track for an early submission to NIH. Besides that I had fun writing together. Its wonderful to be pulled up a bit by someone who is clearly gonna be better than me.