Quality does not decrease during the sprint
What do we mean when the scrum guide says quality does not decrease during the sprint?
The Definition of Done is a commitment made by the Developers. They may renegotiate the scope needed to meet the Sprint Goal, but at no point during the Sprint would they compromise on quality.
If the Developers find that they will not be able to complete all the User Stories selected for the Sprint they may discuss with the Product Owner to leave out some User Stories. This should not anyway have an effect on the Sprint Goal.
This is a good way instead of reducing the quality of you work to complete all User Stories. You should not be cutting corners and saving time by not writing or executing your test cases for example.