For my work I need to downgrade the Inspiron 15 7000 Gaming 7567 which come with windows 10 to windows 7, but after about a year when all the software that I use for work will be updated to windows 10 then I like to re built with windows 10. Is this possible ?
Will the windows 10 product key in the BIOS be loss when I downgrade to windows 7 ?
Will I need a to buy a new windows 10 Key when I re-built the computer ?
It's not an option - with the seventh generation CPU you have, only Windows 10 is supported - there is no support for Windows 7 on this model.
If you need Windows 7, you'll need to return the system and order a different model with a Skylake (6th generation) CPU -- the last one with pre-Windows 10 support. Some Latitude models still offer 6th generation CPUs.
You can always reinstall Windows 10 with no product key, and your digital license recorded with Microsoft will not be affected by installing other OSes in the meantime. However, downgrade rights are not automatic; they typically only come with enterprise licenses and if the OEM specifically provides them, which is not the case here. You would not have a license to install Windows 7 just because you have a license for Windows 10. And as ejn63 said, that PC might not even support Windows 7.