I'm working at an IT service provider. The job is fine, and I would consider my payment as quite good.
In the past months I have been mainly working for a single customer. By my colleagues and managers, they are considered as a very tough customer. They're always very demanding and have withheld payments when they didn't get what they wanted (even if that was way beyond what was agreed before). However, I get along with them quite OK.
A few days ago the customer's manager asked me whether I could imagine working for them. I was hesitant (for the reasons above), but he asked again a few days later, and asked what salary I'd imagine. I then asked for a very high salary (more than twice as much as I earn now), hoping that this would end this discussion.
He came back to me today and said I would get the salary I asked for, also when I would be joining them and a few other contractual details. They told me the reason they wanted to hire me is that at my current company, I also have other duties (working for other customers, and stuff like internal meetings), but they wanted 100% of my time.
I'm unsure whether I really should join them – I actually didn't consider to work for them, that's why I asked for a salary which I thought no-one would accept. Also, I like my current colleagues and managers, and they surely would be irritated if I switched to this "tough" customer. I would most likely continue working with them (on the same project as before, but of course on the other side). This could bring me into weird situations, where I could need to demand much from my former colleagues.
On the other hand, earning more than twice as much as before is a very attractive opportunity. I don't think such an offer would come along any time soon. Also, I checked my current contract, there's no clause in there saying that I cannot work for a customer.
How can I handle such a situation? Also, in case I accept the offer, how can I communicate this to my current colleagues and keep our good relations?