Haaland was in a far, far stronger negotiation position than Bellingham when we signed him. Literally half of the European elite were after him, he already rampaged through the CL with Salzburg and we were in a dire need of a striker that would directly help. This made Dortmund move away from their established stance of not accepting release clauses. Bellingham might have been a top talent (which is why we paid 20+ Mil. € for him), but was still a 16 year old from a Championship side. Make no mistake, the step up from Birmingham to Dortmund was a large one. There is no indication that Bellingham has a release clause in his contract.
Even top talents can not demand everything. Dortmund offers regular CL football, chances for domestic silver ware, competive wages in the talent bracket, a strong history of developing top players from young age and a readiness for compromise for a certain price after a certain time. If they want a release clause on top of that, they can go to a club belonging to a tier below that.
I expect Bellinghams tenure at Dortmund to last at least as long as Sanchos (4 seasons), maybe even a year more than that. I would also be surprised if he does not renew his contract for a slightly longer one in the future as his performances and potential will likely demand a pay raise closer to the clubs top earners. That contract might include a similar gentleman agreement as what existed between Sancho and Dortmund.
Bellingham won´t leave until at least 2023 with the likelier point in time being 2024.