I am leaving my condominium to my brother and sister-in-law when I die. However, if they should sell it, I want $10,000 of what they get for it to go to my son. Do you think there would be any problem with putting this condition in my will?
You can specify nearly anything you want in your will, including your proposed plan. The question, of course, is the feasibility of what you want, and whether it might cause problems down the line. For example, it's unclear who will enforce the provision if your brother and sister-in-law decide not to comply with it. If the matter ended up before a judge, the legal proceedings could easily eat up the $10,000 and possibly much more.
If you trust your brother and sister-in-law to comply with the provisions of your will without the need for enforcement, then you can just tell them what you want in a letter accompanying the will. The letter won't be binding on them, but at least they'll know your intentions.
For more information about what can go in your will, see What a Will Won't Do.