There is a minimum ratio of contract years to bonus money offered. That is why the pop up will say that the bonus money for a contract of x years must be at least y.
As for the other pop up, that is to ensure there is somewhat of a balance between bonus money and salary, so you don't 100% one side or the other and create wacky contracts.
My recommendation when resigning your own players:
*Put the top slider "first year base + bonus" as low as it will go (to the left)
*Put the bonus slider as far left as it will go.
*Put the years to the number of years you want and makes sense for your team.
Young players could get a 4/5/6 year contract if they could be legitimate starters you don't plan on trading/cutting. Older players, you need to be a bit more careful, as mentioned in previous posts about guys retiring. The main thing to remember is that bonus money counts against you when a player is cut, traded or retired, so balance up the amount of bonus money to the likelihood of those events. Likely to be cut/traded/retire, less bonus money, and vice versa.If this initial offer is accepted, bonus! You got the guy for the cheapest contract possible. This could well work for guys rated in the 50's or so, give or take.
However, it's more likely (especially if its a good player), that a pop up appears saying "This players will not negotiate for a bonus less than x"
Whatever the X is from that pop up, write that number down and press ok on the pop up. We'll call this written down number X. Take the X bonus money number and put it in to the Total Bonus box under manual override. Make sure its a completely numerical value, no $ sign and no , either.
The first year base slider will most likely move to the right, that is fine. Both the first year base slider and the bonus % will need to move to the right in order to accommodate the manual bonus amount you just specified. Press submit. If this is accepted, again, happy days, the salary will be as low as possible whilst still being the minimum the player will negotiate for. You could still play around with the first year base + bonus slider, maybe move it to the left a few ticks, but you'll notice that changes your X number so you'll need to re-input it before trying to submit the offer again.
It is possible that you'll see the "bonus cannot make up more than 75% of the first year salary pop up. If that is the case, take the very top slider, the first year base + bonus slider, and knock it to the right a few notches. This will also adjust the bonus money in the manual override box. Take out this new number in the override box and put the original x bonus value that you wrote down originally. All you're doing is shifting some first year money from bonus money over to salary, whilst keeping the total bonus money exactly the same.
If the pop up still appears, just knock the top slider to the right a couple more times, reset the bonus money once more to the minimum the player will negotiate for, and try again.
2 points:
1) Don't worry if you submit an offer then realise something is wrong or crazy out of whack. You can simply withdraw an offer before the next sim by going back to the contract offer screen for the player. There will be a withdraw offer box you can tick at the top, then press submit to withdraw.
2) The info above doesn't hold true for signing free agents, only your own players. Free agents will always sign the offer that has the most bonus money in it, pretty much. Because of this, you need to use a different strategy.
The above probably doesn't make too much sense. I try to be succinct and often fail! If you have any questions, let me know :)