Liquidity can be archived by several means. One is the cash reserves a company has, another are short term credits like the quoted 60 Mil. €, which is basically an overdraft facility on their bank account. They could also put additional shares on the stock market and generate money that way. The last way to ensure liquidity would be loans to bridge over the drought Corvid causes.
Watzkes confidence about the continued financial stability of the club stems mostly from the fact that Borussia Dortmund could easily secure loans with good conditions to keep the club running smoothly for years. In difference to a poster who questioned which bank might even finance Borussia Dortmund right now, they actually would have the pick amongst pretty much any major bank. They have been certificated with an outstanding credit rating (amongst the top 2% companies in the country) and it is pretty obvious why: no current long time loans and high capital with an unusal high equity ratio (over 70%) meaning that they could generate large amounts of money by sales to pay the rates+interest.
What does this mean in relation to a potential Sancho transfer? Not much, actually, just that they are not forced to sell to keep the lights running as they have enough alternative avenues left to secure that.
In the end, it becomes an evaluation what keeping Sancho for an additional year might cost the club (wages, potentially lower fee next Summer, potential interest payments) and if it outweighs the advantages of having him in the squad for that year.
One essential point in this thought process would be the question of an successor to fill the void Sancho will leave behind. There is no doubt in my mind that they would need to sign a relatively high profile player if they would sell right now. The fall off in offensive ability would be too great otherwise and actually put their CL spot at a risk, nevermind being part of the title race. Keeping him for a year would buy time for the various highly talented offensive youngsters (the biggest hopes undoubtly rest on Gio Reyna here) to develop enough so that the compensation of Sanchos departure could happen internally for the most part.