Is it a condition to continue having the pure intention for Allah and concealing the deed from people after completing it, or is the pure and sincere intention before and during the deed sufficient?

Rejecting the intention and regretting a good deed after its completion does not affect it. The rejection of deeds after their completion is invalid and cannot be dispelled. The principles of Sharia (definition) dictate the invalidity of rejecting deeds after their completion, and that the intention of rejection and annulment has no effect whatsoever.

Regretting a good deed is considered among the whisperings of Satan, so a Muslim must repel these thoughts.

Sincerity of intention does not necessitate concealing the deed. The intention can be sincere and the deed apparent, even if concealment is better, as Allah Almighty says: "If you disclose your charitable expenditures, it is good; but if you conceal them and give them to the poor, it is better for you, and He will remove from you some of your sins. And Allah is acquainted with what you do."

If ostentation (riya') afflicts a deed after it was done sincerely, it is said by some that it invalidates it, and by others that it does not. The more appropriate view is that one is rewarded for their deed and punished for their ostentation. Speaking about an act of obedience with the intention of informing people about it is considered blameworthy seeking of fame (sum'ah).