I'd say it's balanced, possibly even weak, assuming that what you mean by "interact" is an object interaction, of which you get a free one with a move or attack action. This presents ways to do something similar within the rules, though not the same exact effect.
Use two weapons
You could switch between a finesse weapon and a reach weapon: the finesse weapon could be a rapier for d8 damage, but swapping it out for a reach weapon requires you to attack using strength or choose a whip, which then limits you to d4 damage. Considering that the average damage (d8 vs d4) is about the same as a d6, then the trade between the weapons is fair and it could be effectively treated as having a single d6 weapon.
Throw your melee weapon
Alternatively, you could homebrew a weapon chain that attaches the weapon to your wrist, allowing you to throw it and then retrieve it. The damage would be that of an improvised weapon (a d4), and the free interaction could be to retrieve the weapon, allowing two attacks in the same turn (though the weapon would then be unready until the next turn) or one attack every turn and the weapon would be constantly ready. The down side here is that an improvised thrown weapon still used strength for its attack and damage. So again, we have a melee damage of up to d8, but a reach/ranged damage of d4.
The fly in the ointment, however, is that it seems that the weapons list goes out of its way limit one-handed dex based reach/range attacks to d4 - whip is reach, dagger is finesse thrown, dart is ranged. Everything else requires two hands (bows and crossbows) or is strength based (polearms for reach and spears/axes/javelins for thrown). This may be a conscious decision on the part of the designers or it might just be random chance. As such, any homebrewing in this area should be carefully considered. It doesn't seem to me that a d6 finesse reach/thrown is utterly broken, but they seem to be curiously omitted, so it seems reasonable that someone considered them to be.
Considering the options available to do this without improvisation, the increased cost and restrictions on the reach weapon effectively make this a weak polearm for dex characters. The cost is quite high compared to the other options above, but as they do not achieve the exact result you desire, it may be worth it.
But don't fool yourself - ultimately, what you have ended up with is a whip that does d6 damage for 50G. The restriction requiring the use of Dex is not really a restriction, as any Strength based character using martial weapons has much better options, though they are burdened with the heavy and two-handed features. And the intended recipient, likely the only person in your campaign to ever use the weapon, suffers no negative impact from the "requires dex" restriction. if this is okay with you, then just simplify the weapon to be d6, finesse, reach and be done with it.
Whatever you decide, make it clear to the player that the weapon is an experiment, and if you decide that it is overpowered (or underpowered, though this seems unlikely), you reserve the right to tweak it as needed.