How to abort one post-doc sub-project politely
Bad news is almost always best conveyed:


Immediately
In person


You cannot change that you probably should have said this earlier, but you can prevent it from being further delayed now.

So go find your colleague and tell them you are not going to be able to collaborate with them. Probably best to apologize to them as part of this conversation, too.

A lot of this will depend on how long ago this agreement was and how long you've been procrastinating saying anything. If we're talking a week, it's less important than if this other post-doc has been waiting for you for a month. If it's been a long time, make sure to apologize for taking so long to tell them, too.