It's very unlikely you will even notice a difference on your scalp, except for the linear scar of course. Eyebrow transplants don't require many grafts so it should have very little impact on your main area of hair.
You wont see any loss of hair volume for the minimal amount of hair follicles need for an eyebrow transplant. But you may want to look at having an FUE procedure done instead of FUT, just because of the fact that it is scar-less and the downtime is very minimal. But no matter what procedure you choose, you will see no loss of volume in the donor area. I hope this helps
It is a small amount of hair used compared to the 100,000 hairs on the average head. The donor area has to be carefully selected for best results. See photo below of before and after.
A donor strip for eyebrow hair transplant might take about 300 grafts or so. This volume of hair will not be detectable from the donor strip and may be taken up by a small reduction of your scalp laxity.