Belt of Giant Strength

Equipped creature has base power and toughness 10/10.
Equip {10}. This ability costs {X} less to activate, where X is the power of the creature it targets.

Alchemy Horizons: Baldur's Gate

Forgotten Realms Commander

Equipped creature has base power and toughness 10/10.
Equip {1}0. This ability costs {X} less to activate, where X is the power of the creature it targets.

Belt of Giant Strength will overwrite any previous effects that set the creature's power and toughness to specific numbers. Effects that otherwise modify the equipped creature's power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.