I recently went to a tech interview. Because I was nervous, I forgot everything. The interviewer said they need a week to take their decision. I know that I am a good programmer, and this is a company I would like to work for. Is it a good idea to write a letter now (before they make a decision) to suggest giving me a lower salary during my probation period?
I really don't recommend this, mostly because it's rather unlikely to lead anywhere because unless the company is making it's hiring decisions purely on a race to the bottom on price why on earth would they hire someone who did poorly on a technical test over a candidate that didn't just to save a relatively small amount of money? But also because working for any real length of time at reduced rate completely undermines any future negotiating power you have in terms of getting wage increases.
Realistically you need to accept that this one was not to be and move on - the only circumstances I would suggest doing otherwise would be if they come back to you with specific feedback about your performance in the technical interview and they don't make it clear that they have hired another candidate. In that case you haven't got a great deal to lose by explaining that you flubbed the test through nerves and asking if you could undertake an alternative interview or exercise to prove that you do in fact have the skills.
Asking for a lower salary almost always reflects poorly on you as a candidate. Chances are that if they were going to hire you regardless of your test scores, they might decide to reject instead because of such a message.
It gives a vibe of desperation, which is not a good thing. The company wants to make sure you are a good match for them; how much you want to work for them only factors into it a little bit. How much they pay you, likewise, usually isn't the key deciding factor. Offering to take a pay-cut before you even get an offer will make them think you are desperate to get the job and are begging them to take you. You do not want to start a job from a position of groveling; the company is looking for a professional, not a lackey.
Should I ask for a lower salary during probation period after an unsuccessful interview?
If the hiring company believes you are not capable of handling the technical aspects of your job (which seems to be your assumption), then offering to accept less isn't going to change that.
Companies don't want to hire the cheapest programmer, they want to hire the best programmer that fits within the role and budget.
Take this as a learning experience. Practice interviewing until you aren't so nervous. Go to your next interview more confident and ace the interview.
If your interview went badly, then lowering your asked salary is extremely unlikely to make them want to hire you anyway. If you're dead-set on trying to salvage the opportunity at this particular employer, you might call them by phone.
As for what you should say during this call, it would be something along these lines.
Hello X. During my interview I don't feel that I was adequately able to convey my technical skills, I was wondering if you had any questions for me on this topic?
Even if you have enough knowlegde but due to some reasons you failed to express your knowlwdge, an interviewer has no options other than the answers/skills you have presented in your interview OR how you express yourself.
Instead of asking for less salary, accept the fact and better start preparation on how to present the skills/knowledge that you have.
If you are 100% sure that you have really messed it up and you will fail to get selected and you could have done better, then One thing you can do is send a Thank You email for the opportunity they had provided and express the reason why you messed up in very short and request for another chance.
Because Here you do not have anything to loose. But if are lucky, You may get another chance.
But never ask for less salary.