This would be the case even if they aren't selling you goods? I'm basically giving this guy 100+ FP in exchange for nothing, so it feels like the favor is mutual, assuming that he'd only accept if he wants to level the GB?
I disagree.
Arc players get most of their points (depending on the level of the building) for free. Through contributions of other arcs locking their top spots at their full value or close to it. By him turning his attention to this building it is going to A) Cost him more than it will cost you and B) take his attention and FP away from what he actually wants to work on.
Please keep in mind that what I'm saying, I'm not saying for you to take personally in any way or is any observation of you in particular what so ever. If you think about it from his perspective such a request could almost seem, self serving and selfish. You have something he needs, you have nothing he needs. To trade with you would end up costing him twice the points. The points put on yours then the extra points put on his to make it profitable for an arc snipe. He's agreeing to help you to get you the BPs you need but in a way that while it doesn't benefit him much (assuming he had no plans to bring his DT up another level) it's not going to cost him as much as trading you would.
If he does the method I do, put your own points on till a arc can come in and break even then you putting points on actually does not lower the cost for an arc to come in and take a spot. That arc needs to account for your points when figuring the lock amount. It will cost him less by the time he's ready to flip but he will still need to front all the points to allow an arc to lock.
There is no real way of knowing what he means other than asking him, which is exactly what I would do if I were you and phrase it like 'How many points would you like me to put on and what position are you aiming for me to end up in?"
You may be under the assumption you'd be putting points on for first place, he may be under the assumption you know your only getting 5th for BPs. There will be no way to know what way he's going to level the building unless you ask. I wouldn't want to see you disappointed for you to put a lot on only for him to call in his arc friends for a lock once he can.
Realistically if someone is looking for BPs they would need to spend well over the typical 90% + base of that position to secure it. Even then unless it is locked it is not secured. He could, with no mal intentions, offer to pay back the difference and then some to someone to come in and lock over that amount to keep costs of the building down. That would be the cost for him as well as your BPs.
Again not trying to come off as rude just trying to explain how leveling buildings trading arc swaps are vastly different than regular swaps and anyone contributing to a GB looking for BPs either needs to go big or go home or they aren't really helping the owner out much. If all you end up giving him is the reward x .9 for a spot he's still spending the exact same amount he would be having arc friends lock than if you hadn't contributed anything.