Match Portrait Background Tone

I have a high-resolution headshot of a woman against a mid-gray backdrop. I simply need the background darkened so it aligns with a second portrait I’ll supply, giving the impression both images were captured in the same studio session.

Here’s what you’ll receive from me:
• The original RAW/JPEG of the current portrait
• The reference portrait whose background tone you’ll mimic (yes, I have it ready)

What I expect back:
• A finished image with the subject untouched and the background shifted from light gray into a slightly darker gray gradient that mirrors the reference
• Two files: a flattened high-quality JPEG for immediate use and a layered PSD (or TIFF) so I can make tiny tweaks later if needed

No other retouching required—just the smooth, natural background transition. Quick turnaround appreciated; let me know your estimated delivery time along with a couple of recent samples of similar background work if you have them.