If they decide to sell Haaland, I'm positive they can get a much better player as part of the deal than Abraham. It would be really dangerous for Dortmund to count on Abraham as their 1st attacking option so in that case they'd either have to bring in an additional striker or try to work out a swap deal with another club.
These player swap deals rarely become reality, they are often simply media speculations. If Haaland leaves, it will probably be a straight cash deal.
Chelsea are trying hard to bring in Erling Haaland, in order to get approval from Dortmund, Chelsea offered Tammy Abraham an additional 150 million
euros to Dortmund. Tammy Abraham is not a bad player, but his level is very far compared to Haaland. As you said player swaps rarely come true,
then it is likely that Dortmund will ask for a money deal if they really want to sell Haaland. The fact that Sebastian Kehl as Dortmund sporting director
said Haaland will remain at Dortmund in the 2021-2022 season, this confirms Haaland will not for sell. So it is very unlikely that Chelsea can bring in Haaland.