First, check whether a Boxer 'mat' is really a mat
A Boxer's coat is so short and lies so flat that genuine matting is uncommon, which is exactly why a lump in the coat deserves investigation before removal. Feel what the tangle is in good light: loose shed hair brushes out easily and dried debris softens with a damp cloth, but a firm lump that moves with the skin or makes the dog flinch may be a skin issue, and combing at it could break the surface.
Treat any hard, fixed, or growing lump as a vet conversation, not a grooming problem. Note its size and place, and do not squeeze, cut, or pick at it. A soft tangle over normal, comfortable skin is the only thing this routine should remove.
Tease a genuine Boxer tangle out with fingers only
If what you found is a genuine loose tangle — most often in the slightly longer hair at the neck, behind the ears, or where a harness has rubbed damp hair after a walk — fingers are the right and usually the only tool a Boxer needs. Pinch the base of the little clump so the skin stays still, then roll and tease the hairs apart from the outside in. Because the hair is short, a real Boxer tangle is small, and it should loosen within a minute or two of gentle work.
Follow with a soft brush or a rubber grooming mitt over the area, which suits a Boxer's short coat far better than forcing a steel comb through it. If the clump resists finger work, dampen it slightly and try again rather than escalating to pulling; hair that will not separate with fingers on a coat this short is telling you it is attached to something more than itself, and that is your signal to stop and look at the skin properly instead. Keep the dog relaxed with calm praise, and let an excited Boxer settle before any close skin work.
Do not cut close to a Boxer's skin to remove a clump
Scissors are a poor choice on a Boxer precisely because the coat is so short: there is almost no hair length to lift a clump away from the body, so blades working on a tangle sit immediately against skin that wrinkles and moves as the dog breathes and shifts. If a stubborn clump must be removed rather than teased out, clippers used by a groomer, with skin held flat and visible, are safer.
Likewise, never shave or clip into a lump whose nature you have not identified, and never try to drain, lance, or squeeze anything found under the coat at home. Keep the area clean and dry, and let a vet examine anything sore, oozing, or not resolving.
Keep a Boxer's short coat mat-free with minutes a week
Prevention for a Boxer is brief: a once-over with a rubber mitt or soft brush each week lifts loose hair before it can clump, and the same pass doubles as your regular feel of the whole body for new lumps or sore patches. Dry the neck and harness line after wet walks and baths, since the rare Boxer tangle starts where damp hair is pressed flat. Check the skin folds and the areas under the collar as part of the same routine, and keep them clean and dry.
Write down where any clump or lump appeared and what it turned out to be, so a pattern — harness rub in one place, a lump worth monitoring in another — is visible over months rather than rediscovered each time. Adding that note for your Boxer in KinsLe through Care Teams means every helper knows which spot to feel for gently and which one is already being watched.