We really aren't comparable to United, not even close. Utd's issue is their propensity to pay inflated transfer fees and wages for mediocre, overrated, or over the hill players; Dortmund can hardly afford their business model i.e. Sancho joined Chelsea after his loan with us because we will not pay what rich PL clubs offer.
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That said, we aren't a serious club if we let players to join our main BL rivals on good terms then welcome them back even if they are in bad shape or form e.g. Kagawa, Sahin, Götze. Football isn't a charity business, especially not in this era where only Barca and RM are realistically able to keep up with PL revenues.