From what I gather from the CBA, a tender on an RFA is simply an offer. An offer is not a binding agreement like a contract because tenders, according to the CBA, are simply a notice to the player. Therefore, tampering does not apply when an RFA continues negotiating with other teams after signing the tender since it is not a binding agreement. After April 21, 2017, an RFA (Butler) can only negotiate with NE. Going further, if he still has not signed his tender by June 1st. NE can designate him as a June 1st tender which will only require them to pay him 110% of his prior year pay or $660,000.
Article 9 Section 2f:
In the event that a Restricted Free Agent has not signed a Player Con-tract with a Club within the Signing Period in the League Year following the expiration of his last Player Contract, and if the Prior Club has not withdrawn the Qualifying Offer, the Prior Club shall be the only Club with which the player may negotiate or sign a Play-er Contract for that League Year. If the player’s Qualifying Offer is greater than 110% of the player’s prior-year Paragraph 5 Salary (with all other terms of his prior year contract carried forward unchanged), the Club may withdraw the Qualifying Offer on June 15 and retain its rights under the preceding sentence, so long as the Club immediately ten-ders the player a one year Player Contract of at least 110% of his prior-year Paragraph 5 Salary (with all other terms of his prior year contract carried forward unchanged) (the "June 15 Tender"). Notwithstanding the foregoing, for any Restricted Free Agent sub-ject to the Right of First Refusal Only Tender under Subsection 2(b)(i)(1) above who has not signed a Player Contract with a Club within the Signing Period, in order to be subject to the June 15 Tender, the Prior Club must by June 1 tender to such Restricted Free Agent a one-year Player Contract of at least 110% of his prior-year Paragraph 5 Salary
(with all other terms of his prior-year contract carried forward unchanged) or extend the Subsection 2(b)(i)(1) Tender, whichever is greater (the "June 1 Tender").