returning to past employer -leaving present employer

I recently started a new job. Got into the new position and after 2 1/2 weeks decided it wasn't going to work out and am going back to my past employer. When I turned in my resignation to my new employer (less than three weeks working) they asked for three weeks -they have some projects and need help. I verbally agreed that I thought I could give them three weeks and didn't put it in writing because I had to check with my past employer (who I'm returning to). Problem is is that I can only give them one week, two max. I went back to my present employer and apologized that I couldn't stay the full three weeks -I could only stay for one. They said no -three. I tried again then offered two. Again, they wouldn't negotiate, stayed firm on three. I really need to go back to my old company and am upset they wouldn't negotiate. Can I leave on bad terms since they are unwilling to negotiate?

Can I provide a letter indicating that I'm going to have to leave on bad terms since they are unwilling to negotiate the three weeks -and state Friday was my last day without getting sued?

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: I recently started a new job.  Got into the new position and after 2 1/2 weeks decided it wasn't going to work out and am going back to my past employer.  When I turned in my resignation to my new employer (less than three weeks working) they asked for three weeks -they have some projects and need help.  I verbally agreed that I thought I could give them three weeks and didn't put it in writing because I had to check with my past employer (who I'm returning to).  Problem is is that I can only give them one week, two max.  I went back to my present employer and apologized that I couldn't stay the full three weeks -I could only stay for one.  They said no -three.  I tried again then offered two.  Again, they wouldn't negotiate, stayed firm on three.  I really need to go back to my old company and am upset they wouldn't negotiate.  Can I leave on bad terms since they are unwilling to negotiate?

: Can I provide a letter indicating that I'm going to have to leave on bad terms since they are unwilling to negotiate the three weeks -and state Friday was my last day without getting sued?