According to SZ, Bellingham and his father gave their word to Dortmund before the Coronavirus that he would join them.
BVB are ready to pay around €25m for Bellingham, who has made 32 appearances in the Championship this season.
This is considered a bargain for a super-talent many Premier League clubs would have paid a lot more for.
The imminent transfer should make Bellingham the most expensive under-18 player in football history.
That record currently belongs to Alexandre Pato, who AC Milan paid €24m for in 2007 when he was 17.