Can a medical retired combat injured veteran working for the Federal Government DoD- Army, buy back their service time to retire early?

You should be able to

Waiving Military Retirement
Most military retirees are barred from receiving credit toward a civilian annuity unless they waive their military retired pay.
You cannot receive credit for any military service in your FERS retirement computation, if you are receiving military retired pay, unless you were awarded the retired pay:
Due to a service-connected disability either incurred in combat with an enemy of the United States or caused by an instrumentality of war and incurred in the line of duty during a period of war, or Under the provisions of Chapter 1223, Title 10, U.S.C. (pertaining to retirement from a reserve component of the Armed Forces).
https://www.opm.gov/fedshirevets/current-veteran-employees/federal-retirement/

and see handbook:
https://www.opm.gov/retirement-center/publications-forms/pamphlets/ri90-3.pdf

I think the reason you are getting conflicting answers is two-fold. The first is terminology. Early Retirement is either an involuntary separation or a voluntary separation when an organization is undergoing a reorganization or RIF. More likely what you are inquiring about is retiring a your minimum retirement age (MRA). Most government employees covered by FERS refer to this as early retirement. You can use the service from your military buy back when retiring at your MRA but depending on whether your medical retirement is tied to a combat injury or combat veteran who was retired for a non-combat injury determines whether you will need to forfeit your military retirement when you buy back the service. Determining your MRA retirement can be difficult because there are normally reductions of 5 percent per year for each year you are under 62 and the age you are eligible is based on the year you were born. As an ACP you see the effects of different retirement dates on the GRB platform on the Army Benefits Center-Civilian. That platform also has retirement calculators that you can use to plug in specific retirement dates as well as benefit elections and projected accrued sick leave which is also taken into account in determining your retirement pay.

Appears you are eligible, BUT, you have to pay for it. There are calculators that will give you an estimate. Under FERS, I had to put out 4% of the base pay total. There was a 3 year grace period on interest for the payback once I hired on. Presuming you have a few years or so of lower level base pay, the "investment" will have a good return on the back end. My $10K buyback is getting me an extra $1300/mo. No brainer.