The German no is more the rule than the exception. However, its level of hardness is based on contextual factors. It can range from a hard to a flexible no. Only through asking what the barriers are to the yes is it possible to discern how hard the German no actually is.
And converting a German no into a German yes requires identifying, addressing, and overcoming the reasons for the no. This can require a lot of time and effort. But as stated before, the German yes is worth fighting for.
A no in the American context is far more the exception than the rule. Americans pride themselves on being a can-do people, of being open, helpful, good neigbors. They believe in cooperation, teamwork, volunteerism. Americans do not feel comfortable saying the word no.
To reject a request out of hand is to negate these values. An American no, therefore, comes in the form of a strongly conditioned yes. The conditions communicate the reasons why yes is not possible.
Germans often misinterpret the conditional American yes as a committed yes. They then draw the conclusion that Americans don‘t keep their word. To be unreliable, even on minor matters, is considered highly negative in the German context. It is seen as a serious character flaw.
Americans often misperceive Germans as born nay-sayers. They unfairly label their German colleagues as unfriendly, uncooperative, not team-players, labeling them as Herr or Frau Dr. No. American organisations succeed based on teamwork. Saying no to a request is saying no to the team, and saying no to success.
Advice to Germans
Your German no is harsh and unfriendly to the American ear. Either take it out of your vocabulary altogether or at least soften it. Explain your reluctance in a more diplomatic way.
State the reasons why you cannot (yet) enter into an agreement. Then give your American colleague a chance to overcome those reasons. Try to negotiate a mutually beneficial agreement. You may need assistance from this same colleague at a later time.
Advice to Americans
Communicate literally with your German colleagues. If you cannot enter into an agreement simply state no. Provide your reasons, communicate regret, but do not pack your no into „wads of cotton.” Your German colleague won‘t break down into tears.
If you are willing to enter into an agreement, state clearly your level of commitment. Parameters can change. Use a percentage: „Sure, Hans, I can deliver that by next Thursday. However, I have a lot going on at the moment. I can guarantee it 75%. Let‘s talk again on Tuesday.“