You should set the regular price in the price attribute and the sales price in the sales price attribute. Promote your products and attract attention to your ads and free listings by setup the sales and regular prices. When your product shows with sales price annotations, your sales stand out and potential customers see how much they’ll save.
Read more here: https://support.google.com/merchants/answer/6324371
And here: https://support.google.com/merchants/answer/9017019
The sales price attribute can be found in the optional attributes field at the bottom of the feed editor.
Alternatively, you could use conditions to use the sales price of your products in the price attribute when there is a sales price, and if not just use the regular price.
Nevertheless, that is not what Google and we would recommend.
Check this video for the alternative method: https://www.youtube.com/watch?v=jVb0uVfKAV4