A what-if parameter is a disconnected table of permitted numeric inputs, often created with GENERATESERIES. A measure reads the selected value with SELECTEDVALUE and supplies a default when there is no single selection. The parameter doesn't change transaction rows by itself. Another measure must use it in the scenario calculation.
Choose a range and increment that match the decision. These exercises use a fictional discount assumption between zero and thirty percent in five-point steps. Store 10% as 0.10, then format the measure as a percentage when you show it. A disconnected parameter should stay disconnected: relating its values to Sales would filter transactions rather than model an assumption. Check the zero case and the default, since both should reproduce the baseline.