Less extreme would be offering family planning in the form of condoms and birth control globally. You don't have to incentivize people to change their behavior, but help them achieve what they already want.
The optics of paying people in a culture not your own to not reproduce are problematic, yes? Besides, we know how to get people to have fewer children. Raise their standard of living.
Flys reproduce until they are at the carrying capacity, and their environment does not have the resources to support more. This is not the case for humans, which are nowhere near their carrying capacity.
The average income in the US is 68K and the average woman has 1.7 children. The average income in Niger is $1k (adjusted for local costs), and the average woman has 6.8 children. Women in the US could have dozens of children, and none of them would starve (literally) due to social safety.
Even if you assumed families are limited by their ability to feed their children, one person not having kid doesn't mean some other family now has more money.
I see no reason why one person not having a child would mean another has more, as resources are not shared, and if they were, most people aren't bound by resources anyways