The German "no" is more rule than exception. However, its level of binding character ranges 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 "no" is.
A "no" in the U.S. context is more exception than rule. Americans pride themselves on being a can-do people, of being open, helpful, good neighbors.
To reject a request is to reject these values. An American "no" often comes in the form of a conditional "yes" signaling the reasons why help is regretfully not possible.
No less irritating for the Germans is the American conditional yes. It uses terms and phrases which communicate to the non-native speaker a positive response.
Although Germans speak English well, few understand the nuances. The more complex and politically sensitive the material discussed, the more subtle is the language Americans use. It is a sign of professionalism and finesse in the U.S. to communicate rejection in a positive way.
The effect? Two parties have an opposite understanding of the interaction. One believes to have entered into an agreement. The other not.
Even worse, Germans may conclude that Americans don't keep their word. To be unreliable is considered highly negative in the German context. It is a character flaw.
Germans are often misperceived as born naysayers. They can come across as unfriendly and uncooperative. Americans seldom consider the possibility that it is the German way of saying "sorry, I cannot commit to that right now."
The attempt is not made to determine through discussion to what degree the "no" might be a different way of communicating a conditional "yes." The danger in this interaction is twofold.
First, a mutually beneficial agreement is not struck. Second, and more unfortunate, the German colleague might be unfairly labeled as a "naysayer." That person may never become aware of how they are misperceived by their American colleagues. They have no opportunity to correct the unfair label as "Herr Dr. No."
Advice to Germans
Your German "no" is harsh and unfriendly for the American ear. Either take it out of your repertoire altogether, or at least soften it. Explain your reluctance in a more diplomatic way. You won't be accused of being a therapist. Enter into a dialogue with your American colleague by stating the reasons why you cannot (yet) enter into an agreement.
Then give that person a chance to overcome your reluctance. Strive to negotiate a mutually beneficial deal, with both having receivables and deliverables. Keep in mind, you may need and want assistance from this very same colleague at a later time.
Advice to Americans
Communicate more literally with your German colleagues. If you cannot enter into an agreement, simply state so. Provide your reasons, communicate regret, but try not to pack your "no" into "wads of cotton," as the Germans say. They won‘t break down into tears.
If you are willing to enter into an agreement, give clear indications to what degree your "yes" is binding. Parameters can change. Use a percentage: "sure, Hans, I can deliver that by next Thursday. But, I have a lot going on at the moment. I can guarantee it 80%. Let‘s talk again on Tuesday."