Had surgery on my foot to remove a mole & went out into the sun without putting sunblock on and now I have sunburn on the scar?

If the scar got sunburned it will likely peel and you will be left with an area of darkening, this is called post-inflammatory hyperpigmentation. Just be aware that scars continue to heal and change in a process called "remodeling" for at least a full year, and this hyperpigmentation will likely last throughout this time. So what you see now will likely change substantially depending on how old it is. The lower legs also heal very slowly in general, from the surgery and also from any other sun damage or trauma caused afterwards. At this point I would use sunblock diligently. The hyperpigmentation can always be treated at a later point with topical creams containing hydroquinone or with laser.