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301 means "permanently moved": it tells G tnat a.com/example.htm can now be found at b.com/example.htm. Everything from the original page - including authority - is to be found somewhere else.

Assuming the 301 is active in .htaccess, deleting a.com pages is irrelevant, as all traffic to them is redirected to b.com before their content is accessed.

Cancelling the 301 and deleting the original pages will mean the old pages (and their authority) are gone (effectively 410, although they'll return 404 if you do nothing but delete them), and the new ones will lose all the current backlinks to a.com pages (and their authority).

Thanks for your advice. So if I want to recover authority of A.com, I should cancel the 301 redirect and delete the original pages?

You can delete the old domain's pages, if your rewrite rules there are capturing the same request and sending them to the same content on the new domain.

BUT. You should examine your logs on the old and new domains to be sure it is rewriting to the intended destination as you expect. You should see requests from A.com visitors get a 301 response and then the same request on B.com with a 200 response. Then you know that requests for your A.com pages are landing on the new page and not being sent to the homepage or some other page. I would remove any sitemaps from the old site, so they are not being seen as duplicate content on B.com.

IF you have changed your mind, and want to keep the old and remove the new, then remove the rewrite rule from the old domain's .htaccess file on A.com and delete the same content on B.com. If things were done as you describe, that would stop sending A.com visitors to the new site. It doesn't sound like that's what you had in mind.

I don't know what you're looking to achieve here, but whatever authority site A gained from those pages (i.e. from their content and the links to them) will be lost to site A as soon as they go somewhere else.

Moving them (redirecting) removes their contribution to site A, and so does deleting them. Redirecting keeps them where previous visitors, backlinks and search-engines can find them. Deleting them and posting them as fresh content on site B simply destroys all the value they had previously earned, and leaves everything to start again from scratch.

If you want whatever authority those pages added to site A to remain on site A, keep the pages on site A. However, not2easy makes another - and major - important point: what do you want your visitors to do?

301 is NOT an SEO tool per se. If a page needs too move for any reason, then 301 is the recommended method to do so. If that redirect is to a different domain you kind of start over again in the eyes of search engines.

301 is best used to "consolidate" such as widget.html > superwidget.html which is primarily for the benefit of USERS, not search engines.

Use sparingly!