The 24-year-old left PNE to join Bristol City in June 2016 after coming to the end of his contract at Deepdale.
Because of his age at the time, North End were able to negotiate a compensation deal rather than him leaving for nothing on a Bosman.
That deal included a sell-on clause should Brownhill be sold by the Ashton Gate outfit, entitling the Lilywhites to a percentage of the fee City receive.
The Robins skipper has been linked with a move in recent weeks, Burnley, Sheffield United and Leicester City having reportedly shown an interest.
During this window, City can ask what price they want for Brownhill.
However, in the summer when he will be in the last 12 months of his contract there is a clause which allows him to move for £7m.
What percentage of the fee North End negotiated is not clear.
Sell-on clauses are common practice. Port Vale got a percentage of the fee when PNE sold Jordan Hugill to West Ham United two years ago.
Likewise, Aston Villa had a sell-on clause with Callum Robinson who left North End last summer to join Sheffield United.
PNE signed Brownhill for the academy after he was released by Manchester United as a schoolboy.
He quickly impressed in the youth team and forced his way into the first-team squad at the age of 17.
After two substitute appearances, he scored on his first start for North End in a 2-1 win against Gillingham in October 2013.
Brownhill went on to play 64 games for PNE, scoring six goals.
In the second half of the 2014/15 promotion season, he lost his place in midfield.
When games did not come his way in the next campaign, Brownhill joined Barnsley on loan in January 2016.
Lee Johnson was manager of the Tykes at the time before leaving for Bristol City.
It was at the end of that season that Johnson moved for Brownhill’s services after he had run down his contract at Deepdale.
He has developed into a key player for the Robins, making 160 appearances so far and netting 17 goals.
At the start of the season Brownhill was appointed City skipper.