If you want any products to display the original price (slashed out) and then show a promotional discounted price, then you can use the Specials feature.

You can find this in Catalog > Products > Click on any product > Edit > Specials tab.

Input the discounted price in the price field.
If you want to have different discounted price at different time, you can input the priority and dates.

If not, just leave the priority as "1".

If you set the price $50 in Data tab, then you can set a lower price tag at Specials tab, like $30.

Then input the date range that you want to sell at this price.
If there's no time limit, just set a super far end date like yeat 2299.

Click on Save.