Germany: tax refund for a limited (non-permanent) German residence permit

I have a non-EU nationality and I have a limited (not permanent) German residence permit (Aufenthaltstitel) which is valid until the end of February 2020. However, I will unsubscribe I from GermanyAbmeldung) in a week and go to my home country since my study here is over. Therefore, I will only stay here for about 1 week. Three days ago, I bought something quite expensive and got a tax-free form from Global Blue (GB) that has NO been sealed by custom (Zoll) still. At first, when I arrived at one of the GB offices here, the officer said that she can help me reimburse the tax-free amount, even without the customs stamp, but then I must inform customs at the airport once Fly back to my home country. However, when he saw the residence permit in my passport, he wasn't sure if this works. Therefore, I recommend that you ask first the custom at the airport tomorrow (explain my situation), and once I get the seal, I can even go back with it and recover the money. As the airport is quite far from the place where I am staying, I would like to ask about your experience, is it possible to get a tax-free refund? Thank you.